The present invention relates to a variable capacitance integrated electronic circuit module which is digitally controlled. It also relates to a variable frequency oscillator which comprises such a module, and also a mobile telephone terminal which comprises the oscillator.
Many electronic circuit applications require implementing a capacitor assembly with variable value. In particular, the capacitor assembly may be digitally controlled, so that its capacitance value is controlled by a digital control code. The capacitor assembly is then electrically equivalent to an electric condenser which has a capacitance corresponding to the digital control code. When this control code varies, its variations produce switching operations within the capacitor assembly, so that the capacitance value itself varies in response to the variations in the control code.
FIG. 1 is a layout diagram of a digitally controlled variable capacitance integrated electronic circuit module of the prior art, on a circuit substrate. The module, indicated in all drawings by reference number 100, comprises a set of basic cells 10 which each have a functional block 11 and a control block 12, with a control junction which connects the control block 12 and the functional block 11 of the same cell. The functional block 11 is capable to be switched between multiple, generally two, individual capacitance values. The functional blocks 11 and the control blocks 12 are distributed on the surface of the integrated circuit substrate in a matrix arrangement, which is formed of columns and rows of juxtaposed blocks. X designates the direction of the rows and Y designates the direction of the columns. The directions X and Y are respectively horizontal and vertical in the figure.
In a known layout, the functional blocks 11 on one hand and the control blocks 12 on the other hand are distributed in separate, successively alternating columns. Thus a column of functional blocks 11 is intermediate between two columns of control blocks 12 and vice versa. The module 100 additionally comprises:                two output paths, respectively labeled 1 and 2, which are each formed of path segments placed above the substrate and parallel to the direction X of the rows of the matrix arrangement; and        two power supply paths, respectively labeled 4 and 5, which are also placed above the substrate.        
Each functional block 11 of the module 100 has two output terminals, which are respectively connected to a segment of one of the two output paths 1 or 2, and to a segment of the other output path. In other words, the functional blocks 11 of all the basic cells 10 are connected in parallel between the two output paths 1 and 2, so that their individual capacitance values are added to produce the capacitance value of the module 100. The variable capacitance of the module 100 is therefore produced between the two output paths 1 and 2. In addition, the segments of output paths 1 and 2 alternate between one and the other of the two output paths along the direction Y of the columns.
Furthermore, each control block 12 has two power supply terminals which are respectively connected to one and the other power supply path 4 and 5. These two power supply paths 4 and 5 can be respectively connected to a positive terminal labeled Vcc of a power source for the module 100 and to a ground terminal labeled Gnd.
In a common layout, the two power supply paths 4 and 5 are each formed of path segments arranged in parallel to the direction Y of the columns in the matrix arrangement. In addition, these power supply path segments can be superimposed in different metallization levels above the substrate of the module 100, with the segments of path 4 being in line with the segments of path 5. For this reason, the segments of the power supply paths 4 and 5 appear intermingled in FIG. 1.
As an example, FIG. 2 is a circuit diagram of a possible embodiment of the basic cell 10 for such a digitally controlled variable capacitance integrated electronic circuit module. The operation of such basic cells is assumed to be known to a person skilled in the art, and is not described in detail here. The reference number 6 indicates the control junction connecting the control block 12 and the functional block 11 of one and same cell 10. The segments of the power supply paths 4 and 5 again appear superimposed in this figure. FIG. 2 also shows that each control block 12 comprises a doped well which is formed in the substrate of the module 100. For illustrative purposes, this doped well is assumed to be an N-type well and is labeled N-well. The letters R, C and H respectively indicate three control bits of the module 100, which together form the digital control code for each basic cell of the capacitance of the module 100.
In a known manner, when the capacitance value of the module 100 is ordered to increase progressively, the basic cells 10 are switched in a determined order. Advantageously, this switching order corresponds to a winding path through all functional blocks 11 in the matrix arrangement of the module 100. In FIG. 1, this path is symbolized by the line with arrows labeled T. It is composed of a continuous path traveling back and forth in parallel to the direction X of the rows of the matrix arrangement, and progressively offset in parallel to the direction Y of the columns.
The segments of output paths 1 and 2 are also distributed in the metallization levels which are superimposed on the substrate of the module 100. In the arrangement in FIG. 1, each segment of the output paths 1 or 2 successively crosses all segments of the power supply paths 4 and 5, when projected into a plane parallel to the surface of the substrate. Each intersection is the site of capacitive interaction between the power supply path segment and the output path segment concerned. This capacitive interaction is represented in FIG. 2 by a capacitor symbol of dotted lines designated by the letter P. Four capacitances from path segment intersections are then added to the individual capacitance value of the functional block 11 of each basic cell 10, and in particular to the minimum individual capacitance value of this cell. Because of this interaction between output paths and power supply paths, it is not possible to obtain a minimum capacitance value of the module 100 which is less than 4.2 pF (picofarad). When such a module 100 is associated with an inductor within a variable frequency oscillator, the minimum capacitance value of the module 100 determines the maximum oscillation frequency which is possible for the oscillator. For example, with an inductor of 0.7 nH (nanohenry) and a variable capacitance module 100 as has just been described, the oscillator cannot produce a periodic signal at a frequency of 4 GHz (gigahertz). However, there are more and more devices such as mobile telephone terminals which require oscillators capable of producing very high variable frequency signals of 4 Ghz or more.
In addition, each functional block 11 contains doped areas, in particular positively doped areas, which must be separated from the N-wells of the control blocks 12. The alternation of positively doped areas and N-wells in the substrate of the module 100, along the direction X of the rows of the matrix arrangement, prevent closer placement of the columns. For this reason, the level of integration of the variable capacitance module cannot be increased beyond a limit value.
A first object of the invention is therefore to realize variable capacitance integrated electronic circuit modules which have a minimum capacitance value of less than 4.2 pF.